Why doesn't Tris regret joining Dauntles?

In Chapter 37, Caleb, Marcus, and Andrew Prior accompany Tris to Dauntless, and for everyone but Tris, jumping onto the moving train proves very difficult. Marcus assumes she now regrets choosing Dauntless, but Tris denies it, saying there were some things she needed to learn - including how to be selfless, as selflessness is bravery.